In operation, computers, radios, transmitters and other electronic systems generate electrical signals in the form of radio frequency waves or electromagnetic radiation or both. If not properly shielded, these signals can interfere with the operation of unrelated equipment. Radio frequency interference ("RFI") is interference from sources of energy outside a system. Electromagnetic interference ("EMI") is interference, generally at radio frequencies, generated within the system. To prevent EMI/RFI interference, governmental regulations and industry standards require that the sources of radio frequency or electromagnetic radiation within a system be shielded.
At the same time, sensitive electronic components must also be protected from more tangible forms of disruption. Other sources of disruption to the operation of electronics systems are moisture and foreign particles, such as dust. The presence of either of these environmental hazards can corrode or otherwise debilitate electrical contacts.
One method of protecting such systems from EMI/RFI interferences and exposure to moisture and dust is to place the equipment in a shielded cabinet. To provide adequate protection, the cabinet is preferably designed to eliminate grooves, crevices, and other openings which allow passage of RF waves and which accumulate and channel moisture and debris into the interior of the cabinet. The top of a cabinet is a notorious site for the accumulation of moisture and debris which subsequently tend to find their way into the cabinet. Conventional cabinets generally utilize a top construction having a top panel secured between the frame members of the cabinet. This provides a uniform flat surface across the top of the cabinet. Unfortunately, this juncture between the top panel and the frame members also provides a channel that is difficult to seal and does not inherently inhibit the passage of RF waves, moisture or debris. An example of such a cabinet construction is found in U.S. Pat. No. 4,768,845.
There exists a further problem with respect to transporting these cabinets. Most cabinets are designed to remain in one location and are not designed with ease of transportation in mind. In order to transport a cabinet, it is often necessary to place straps or other securing devices around the cabinet structure. Other methods include inserting eyelets through the panels and frame members. The first technique has the disadvantage of placing stress on the cabinet which it may be ill-designed to resist. The second technique has the disadvantage of violating the integrity of the structure, thus providing openings which can permit EMI/RFI waves to pass and which can admit dust and moisture into the cabinet. Therefore, there exists a need for a cabinet which minimizes the passage of RF waves, moisture and debris, and which may be easily transported without stressing or violating the cabinet structure.
Another problem inherent in the design of cabinets concerns junctures or joints between components which form the cabinet. Such joints may be found between the cabinet frame and a door, or between the frame and panels connected to the frame. Solid metal cabinet components provide EMI/RFI shielding and protect against the intrusion of moisture and debris. However, each juncture between these components provides openings through which RF waves, moisture and debris may pass.
Several attempts have been made to provide a gasket for placement within these junctures to provide EMI/RFI shielding and to seal against the intrusion of moisture and debris. For example, U.S. Pat. No. 4,652,695 discloses a clip-on shielding strip designed to provide EMI/RFI shielding and which may be placed in the junctures between cabinet components. The device disclosed in the '695 patent utilizes a conductive mesh positioned around a resilient core. The core and mesh are secured to a metal clip with a clamping flange which clips to the mesh. The metal clip conductively contacts the cabinet frame and the conductive mesh conductively contacts to a side or door panel. This design has several disadvantages. The metal clip prevents this gasket from being fastened to curved surfaces and corners. Another disadvantage results from the juncture between the mesh and the clip, which juncture increases the resistance and thus adversely impacts the conductivity between the mesh and the metal clip. Additionally, this gasket provides a metal-to-metal interface which does not provide an effective seal against the passage of moisture and debris. Further, the metallic mesh is susceptible to a further reduction of conductivity by the accumulation of moisture and dirt on the mesh.
Another attempt to solve these problems is disclosed in U.S. Pat. No. 4,659,867. The '867 patent teaches a gasket having a conductive tubular element formed of an extruded, resilient elastomer conductively bonded to a U-shaped metal clip. This design attempts to provide EMI/RFI shielding and to seal against water and debris. However, since the same material is used for both the shielding and sealing functions, both are compromised. The elastomer does not provide as effective a barrier to moisture or debris as does a conventional rubber gasket and is more expensive. Furthermore, conductivity may be decreased by the accumulation of moisture and debris on the surface of the elastomer and by the resistant interface between the elastomer and metal clip. Additionally, the metal clip is not well suited for use on curved surfaces and corners.
Therefore, despite the various efforts found in the prior art, there remains a need for an improved gasket which can effectively shield against EMI/RFI and which can also provide an effective seal against moisture and debris.